arch/xtensa: General cleanup, remove dead code

There was a bunch of dead historical cruft floating around in the
arch/xtensa tree, left over from older code versions.  It's time to do
a cleanup pass.  This is entirely refactoring and size optimization,
no behavior changes on any in-tree devices should be present.

Among the more notable changes:

+ xtensa_context.h offered an elaborate API to deal with a stack frame
  and context layout that we no longer use.

+ xtensa_rtos.h was entirely dead code

+ xtensa_timer.h was a parallel abstraction layer implementing in the
  architecture layer what we're already doing in our timer driver.

+ The architecture thread structs (_callee_saved and _thread_arch)
  aren't used by current code, and had dead fields that were removed.
  Unfortunately for standards compliance and C++ compatibility it's
  not possible to leave an empty struct here, so they have a single
  byte field.

+ xtensa_api.h was really just some interrupt management inlines used
  by irq.h, so fold that code into the outer header.

+ Remove the stale assembly offsets.  This architecture doesn't use
  that facility.

All told, more than a thousand lines have been removed.  Not bad.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>